1. Field of the Invention
The present invention relates generally to a method of encasing electrolytic capacitor stacks or wound rolls, and more particularly to a method of vacuum forming a plastic sheet over an electrolytic capacitor stack or wound roll to form an external package.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate may be rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in a metallic tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in U.S. Pat. No. 5,131,388.
Since these capacitors must typically store approximately 30-40 joules, their size can be relatively large, and it is difficult to package them in a small implantable device. Currently available ICDs are relatively large (over 44 cubic centimeters (cc)), generally rectangular or cylindrical devices about 12-16 millimeters (mm) thick. A patient who has such an implantable device may often be bothered by the presence of the large object in his or her pectoral region. Furthermore, the generally rectangular or cylindrical shape can in some instances lead to pocket erosion at the somewhat curved corners of the device. For the comfort of the patient, it is desirable to make smaller and more rounded ICDs. The size and configuration of the capacitors has been a major stumbling block in achieving this goal. In ICDs, as in other applications where space is a critical design element, it is desirable to minimize the wasted volume in the capacitor package.
Conventional capacitor cases using metallic cases are generally known, such as those disclosed in U.S. Pat. No. 5,522,851 issued to Fayram, however significant reductions in capacitor package volume have not been possible with such metal capacitor cases. Metallic capacitor cases act as conductors carrying a negative charge. Thus, edge margins providing spacing between the metallic case and positively charged anodes are necessary to prevent electrical connection between the metallic case and anodes, wasting volume and increasing the size of the capacitor package. Furthermore, metallic cases of reduced dimensions must be machined to very precise tolerances, prolonging new product development time and assembly time. For these reasons, capacitors using such metallic cases are particularly expensive. Thus, there is a need for an improved method of encasing capacitor stacks or wound rolls.
The present invention is directed to a method of vacuum forming a plastic sheet over an electrolytic capacitor stack or wound roll to form an external package and to electrolytic capacitors made according to such method.
In one embodiment, a thin plastic sheet is heated to its soft transition temperature. The heated plastic sheet is then draped over an assembled capacitor stack or wound roll. Next, a vacuum is applied, causing the plastic to pull tightly around the capacitor stack or wound roll. The plastic is then allowed to cool to form a capacitor case. According to the present invention, a frame may be used to secure the plastic sheet during heating and a support fixture may be employed to support the assembled capacitor stack or wound roll while the plastic sheet is draped over and formed to the capacitor stack or wound roll.
The method of the present invention minimizes wasted volume in the capacitor package and minimizes capacitor assembly time. The volume of the capacitor package is minimized because the plastic case is formed directly onto the internal functioning components of the capacitor. Furthermore, since the plastic case is not a conductor, additional space is saved as no edge margin, or spacing between the capacitor and the case, is required. Additionally, whereas in most vacuum formed articles a male form tool must be used to form the plastic sheet into the desired shape and then the product is inserted into the resulting cavity, forming the plastic sheet directly onto the capacitor according to the present invention can be done without the use of a male form tool, further decreasing assembly time and cost.